Tritiated Thymidine Internalization in Zebrafish Early Life Stages: Joint Use of Experimental Procedures and Microdosimetry.

Tritium is found in the environment under three forms: free in the water, gaseous, and bound to organic matter. Once internalized in living organisms, it can be found in two forms: tissue free water tritium (TFWT) and organically bound tritium (OBT). This study aims to better understand OBT internalization in living organisms and to show the complementarity between experimental procedures and microdosimetry simulations that have often been used to obtain more information on imparted energy to cell nuclei. To do so, tritiated thymidine, an organic form of tritium, was chosen and zebrafish embryos [3.5 h post fertilization (hpf)] were exposed to a range of activity concentrations (2.21 × 103 to 5.95 × 105 Bq/mL). First, individual zebrafish embryos were sampled after different exposure times (1 to 96 h) to qualify the internalization kinetics. Then, the barrier role of the chorion was assessed after 2 days of exposure. Lastly, individual zebrafish embryos were sampled after 1 and 4 days of exposure to measure the internalization in the whole fish and its DNA, but also to highlight a possible link between the internal dose rate and the external activity concentration. Microdosimetry simulations were also made to quantify the imparted energy that could occur in the zebrafish cells after exposure to tritium. Results showed that when bound to thymidine, tritium rapidly incorporates in zebrafish early life stages, with the internalization being almost complete after 24 h. Results also showed that while the chorion acted as a barrier to prevent thymidine from entering the embryos, significant levels could still be measured in the whole organisms as well as in DNA. This study also highlighted that when the external activity concentration increased, the internal dose rate increased as well, following a sigmoidal trend. Microdosimetry simulations highlighted that the size and shape of the cell matters, and that the smallest cells seem to be at the greater risk, with only low-energy electrons inducing energy depositions. A linear fit was also found between the mean energy deposited and the logarithm of the radius of the cell, thus showing that the quantity of deposited energy is proportional to the radius of the cell. While this study highlighted important internalization pattern, it will also be used as the starting point of a study focusing on the toxic effects of tritiated thymidine on zebrafish in its early life stages.